The present invention relates to delay handling in modulator loops. More specifically, the present invention provides a filter in the modulator loop which compensates for delays introduced by, for example, a power switching stage or an output filter.
With pulse width modulation (PWM) and other modulation techniques, the delay introduced by switching and output filter stages must be effectively dealt with to alleviate the adverse effects such delays have on circuit stability. This is particularly true for modulators which have relatively high power switching stages because the delays can become very large with respect to the pulse repetition frequency of the loop. A traditional solution to the problem of delay handling will be described with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram of a typical modulator loop 100. The logic output of modulator 102 drives an inverting power stage 104 the output of which is filtered by output filter 106. A feedback resistor 108 and attenuation resistor 112 are provided for the purpose of introducing negative feedback from the output of the loop to modulator 102. FIG. 2 shows two waveforms 202 and 204 from the modulator loop of FIG. 1 without delay compensation. As shown, due to the delay introduced by the switching and filter stages, the positive swing of waveform 204 (i.e., the output filter 106) is nearly in phase with the positive swing of waveform 202 (i.e., the logic output of modulator 102).
Thus, where the original design of the loop contemplates negative feedback, the delay converts it to positive feedback and therefore loop instability (not shown) results. For this reason, a filter capacitor 110 is provided in parallel with feedback resistor 108 for delay compensation. Capacitor 110 produces a zero in the feedback loop, effectively bypassing the attenuation caused by resistors 108 and 112 for high frequencies.
Unfortunately, because this type of delay compensation is performed after the delay has been introduced into the loop, it is difficult to correct all of the delay""s negative consequences with regard to loop stability. In fact, this type of compensation technique has had only limited success and, as a result, has limited the delay tolerance and the overall performance of today""s modulators. One solution is to use feedback signals directly from the logic output of the modulator, i.e., before the delay is introduced, in combination with the output of the power stage and/or the output filter. Unfortunately, while the output of the modulator (202) and the filter output (204) have similar characteristics there are significant differences in content in the modulator loop""s frequency range of interest due to the non-ideal nature of the power stage as discussed in commonly assigned U.S. Pat. No. 5,974,089 for METHOD AND APPARATUS FOR PERFORMANCE IMPROVEMENT BY QUALIFYING PULSES IN AN OVERSAMPLED, NOISE-SHAPING SIGNAL PROCESSOR issued on Oct. 26, 1999, the entirety of which is incorporated herein by reference for all purposes. This makes it very difficult to achieve high fidelity operation while feeding these signals back.
In view of the foregoing, it is desirable to provide an improved technique for compensating for delays in modulator loops such that greater delays may be tolerated without adversely affecting loop stability.
According to the present invention, a feedback technique for modulator loops is introduced which addresses the difficulties discussed above. The feedback technique described herein uses the output of a low voltage modulator stage to compensate for the delay introduced by subsequent power and filter stages while, at the same time, achieving a high level of fidelity notwithstanding the differences in signal content as discussed above. The invention achieves this result by filtering the output of the modulator stage such that its frequency components outside of the modulator loop""s frequency range of interest are transmitted to the feedback path while its frequency components inside the loop""s range of interest are attenuated. Thus, the stability of the loop is enhanced due to the feeding back of some portion of the modulator output, while the fidelity of the output spectrum of the loop is not adversely affected by undesirable modulator output components within the range.
Depending upon the type of modulator, the frequencies attenuated and transmitted by the feedback filter of the present invention vary. For example, for a baseband modulator, the filter attenuates frequencies in the baseband and transmits higher frequencies such as, for example, a high pass filter. For a band pass modulator, the feedback filter may behave like a notch filter, attenuating frequencies within the relevant band. Similarly, for a band reject modulator, the filter may behave like a band pass filter, while for a high pass modulator, the filter may behave like a low pass filter.
In a modulator loop which tolerates a 250 ns delay without the improvements of the present invention, the addition of the feedback technique described herein has been shown to increase the delay tolerance level to greater than 500 ns. This means that, according to the present invention, very large power devices may be employed with low voltage modulators despite the troublesome delays associated with such power devices. For example, a specific embodiment of the invention is capable of delivering more than 1000 W ( greater than 1 hp!) into a 4 ohm load with very high fidelity. For a 1 ohm load (e.g., an industrial motor) this is the equivalent of more than 5 hp! This is a significant improvement over currently available high fidelity modulator loops and is sufficient for many relatively high power industrial applications.
Thus the present invention provides a modulator loop having an associated band pass frequency range and including a switching stage having a first delay associated therewith. The modulator loop also includes a modulator stage having a feedback input. The output of the modulator stage is coupled to the input of the switching stage. A first feedback path is coupled between the output of the switching stage and the modulator stage. A notch filter corresponding to the band pass frequency range is coupled between the output of the modulator stage and the feedback input of the modulator stage for compensating for the first delay.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.